var Base64={
   
  enKey: 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/',
   
  deKey: new Array(
    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 62, -1, -1, -1, 63,
    52, 53, 54, 55, 56, 57, 58, 59, 60, 61, -1, -1, -1, -1, -1, -1,
    -1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14,
    15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, -1, -1, -1, -1, -1,
    -1, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40,
    41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, -1, -1, -1, -1, -1
  ),
   
  encode: function(src){
    //用一个数组来存放编码后的字符，效率比用字符串相加高很多。
    var str=new Array();
    var ch1, ch2, ch3;
    var pos=0;
    //每三个字符进行编码。
    while(pos+3<=src.length){
      ch1=src.charCodeAt(pos++);
      ch2=src.charCodeAt(pos++);
      ch3=src.charCodeAt(pos++);
      str.push(this.enKey.charAt(ch1>>2), this.enKey.charAt(((ch1<<4)+(ch2>>4))&0x3f));
      str.push(this.enKey.charAt(((ch2<<2)+(ch3>>6))&0x3f), this.enKey.charAt(ch3&0x3f));
    }
    //给剩下的字符进行编码。
    if(pos<src.length){
      ch1=src.charCodeAt(pos++);
      str.push(this.enKey.charAt(ch1>>2));
      if(pos<src.length){
        ch2=src.charCodeAt(pos);
        str.push(this.enKey.charAt(((ch1<<4)+(ch2>>4))&0x3f));
        str.push(this.enKey.charAt(ch2<<2&0x3f), '=');
      }else{
        str.push(this.enKey.charAt(ch1<<4&0x3f), '==');
      }
    }
    //组合各编码后的字符，连成一个字符串。
    return str.join('');
  },
   
  decode: function(src){
    //用一个数组来存放解码后的字符。
    var str=new Array();
    var ch1, ch2, ch3, ch4;
    var pos=0;
    //过滤非法字符，并去掉'='。
    src=src.replace(/[^A-Za-z0-9\+\/]/g, '');
    //decode the source string in partition of per four characters.
    while(pos+4<=src.length){
      ch1=this.deKey[src.charCodeAt(pos++)];
      ch2=this.deKey[src.charCodeAt(pos++)];
      ch3=this.deKey[src.charCodeAt(pos++)];
      ch4=this.deKey[src.charCodeAt(pos++)];
      str.push(String.fromCharCode(
        (ch1<<2&0xff)+(ch2>>4), (ch2<<4&0xff)+(ch3>>2), (ch3<<6&0xff)+ch4));
    }
    //给剩下的字符进行解码。
    if(pos+1<src.length){
      ch1=this.deKey[src.charCodeAt(pos++)];
      ch2=this.deKey[src.charCodeAt(pos++)];
      if(pos<src.length){
        ch3=this.deKey[src.charCodeAt(pos)];
        str.push(String.fromCharCode((ch1<<2&0xff)+(ch2>>4), (ch2<<4&0xff)+(ch3>>2)));
      }else{
        str.push(String.fromCharCode((ch1<<2&0xff)+(ch2>>4)));
      }
    }
    //组合各解码后的字符，连成一个字符串。
    return str.join('');
  }
};